The present invention relates to a method for producing large diameter steel pipes having excellent shape, toughness and tensile strength.
In recent years, along with increased demand for petroleum and natural gas, increased remoteness of their sources, and location of their sources in polar and colder areas, pipe lines for transporting these materials have been designed for low-cost and mass-transportation so that general tendencies have been toward enlargement of pipe diameter and high-pressure qualities and thus increased diameter and wall thickness of the pipes and increased tensile strength and toughness of the pipes have been demanded and with less residual internal stress, and excellent pipe shape as regards pipe straightness and roundness have been demanded.
It is now impossible to produce large diameter steel pipes having excellent shape, toughness and tensile strength to meet the above demands by conventional producing processes such as the UO process, the cage forming process, the spiral forming process, etc., and various production methods have been invented but none of them has ever been successful. Some of them will be explained hereinunder.
Japanese laid-open patent specification Sho 48-36014 (filing date: Sept. 8, 1972, laid-open date: May 28, 1973 and hereinafter called Invention A) discloses a method for treating a steel pipe of large diameter which comprises arranging an induction heating coil around the pipe, moving the pipe and the coil mutually to heat the pipe locally with a low frequency alternating current at a temperature not lower than the transformation point, cooling the pipe by means of a spray rapid cooling device surrounding the pipe, and tempering the pipe by means of an induction heating coil arranged around the pipe.
Also a Japanese Patent Publication (Filing date: Feb. 1, 1964; publication date: Mar. 22, 1967, hereinafter called as Invention B) discloses a method which comprises expanding a steel pipe produced by forming and welding, heating the pipe locally at a temperature between 650.degree. and 1000.degree.C by means of a coil and a low frequency induction current, and forcedly cooling as well as tempering the heated portions of the pipe.
However, it is impossible to obtain an excellent large diameter steel pipe which is the object of the present invention by the prior inventions A and B. This is because when heat treatment is carried out according to the prior inventions A and B, the following problems take place as compared with the pipe before the treatment.
1. The roundness of the pipe is remarkably inferior. PA1 2. The straightness of the pipe is remarkably inferior. PA1 3. Remarkably large residual stress exists. PA1 4. The circumferential tolerance is remarkably bad.
1. Explanations will be given relating to problem (1). When a steel pipe is cooled uniformly under a restraining condition, the roundness of the pipe does not vary so far as no deformation due to local heating is caused. However, the length of steel pipes produced by an ordinary method is about 20 meters at the longest, and then the steel pipes are subjected to heat treatment one by one. In this case, the ends of the pipe are subjected to small restricting condition as compared with the other portions of the pipe, and are greatly deformed by external stress and internal stress. Therefore, when a steel pipe is expanded enough to fully satisfy the standard of API (Americal Petroleum Institute) as shown in Table I and then subjected to heat treatment according to the prior invention B. The size and shape of the pipe is severely deteriorated. In addition, the steel pipe with rolling scale and heating scale with thereon is treated no special cooling means in the invention B, so that remarkably irregular cooling effects are caused and thus the steel pipe made according to the invention B does not meet the desired standards very well.
2. Regarding the straightness problem (2), it is of course desirable to prevent the deformation due to gravity by localizing the heating and the cooling as much as possible, as described in the prior invention B. However, according to the results of tests conducted by the present inventors, the deformation due to gravity matters only when the length of the pipe heated is 5 times or more the pipe diameter (D), and it does not matter so much so far as the ordinary magnetic induction heating method is applied. It is rather whether or not the starting temperature of cooling, the volume of coolant and the finishing temperature of cooling are uniform across the cross section perpendicular to the axis of the pipe that determines the straightness of the pipe, and this has almost no relation with the straightness of the pipe before the treatment. In other words, it is necessary that the above three factors be maintained as uniformly as possible within the same cross section. The prior inventions A and B give no consideration to these conditions.
The residual stress which is caused during the quenching and tempering treatments and remains afterward should be as small as possible because a practical problem will occur in the respect of the following three points, if large residual stress exists within the finished steel pipe.
1. When the pipe is cut to a given length, the cut portion will have bad shape due to expansion or contraction.
2. The bending characteristics are reduced.
3. The fracture resistance is reduced.
When the steel pipe is forcedly cooled in the quenching or tempering treatment, large residual stress occurrs if only the outer surface is cooled as shown in Table 3. This residual stress is also one factor causing deterioration of the roundness and the straightness.
With regard to point (3) above, when the steel is quenched according to the method A, etc., the diameter of the outer circumference of the steel pipe is increased due to the transformation characteristics of the steel material and the wall thickness decreases. For this reason, even when the outer circumference is equalized along the entire length of the pipe by an expansion treatment prior to the quenching, the outer circumference change is not uniform due to the difference in the transformation characteristics caused by differences in the quenching conditions and the degrees of restraint at different points along the length of the. Particularly in case of quenching treatments of large diameter and thin walled steel pipes having low hardenability, the above tendencies are remarkable. Then after the quenching, the steel pipe is subjected to tempering for the purpose of adjusting the strength level and improving the toughness. In this case, the pipe diameter which has been enlarged by the expansion decreases depending on the tempering temperature. As explained above, the expansion treatment is necessary as in the present invention because the diameter change is not constant but varies depending on the treating condition even when the quenched and tempered steel pipe has been expanded to make the outer circumference uniform prior to the treatments.
The above explanations are true in connection with the center portion of the steel pipe which is subjected to a uniform restraint during the treatment, but, as is very obvious to the person skilled in the art, in the case of pipe portions such as the end portions which have are less restrained as compared with the center portion, the pipe diameter increases remarkably and the end of the pipe is formed into a trumpet-like shape. This can be prevented to some degree by controlling the restraint applied to the end portions and the cooling conditions, but in this case the qualities of the material in these portions become substantially different from those of the other portions due to the change of the cooling conditions at these portions, and thus no industrial advantage is obtained.
Therefore, one of the objects of the present invention is to overcome the above defects and to provide a method for producing a large diameter steel pipe having excellent shape as well as excellent toughness and tensile strength.
The features of the present invention lie in a method for producing a large diameter steel pipe having excellent shape, toughness and tensile strength which comprises subjecting the pipe to descaling and to heat treatment which comprises heating the steel pipe locally successively from one end to the other end of the steel pipe in the axial direction and forcedly the pipe, and expanding the steel pipe after the heat treatment.